In an attempt to increase the efficiencies and performance of contemporary jet or gas turbine engines such as those used in industrial, marine or vehicle applications, engineers have progressively pushed the engine environment to more extreme operating conditions. The harsh operating conditions of high temperature and pressure that are now frequently specified place increased demands on engine components and materials. Indeed the gradual change in engine design has come about in part due to the increased strength and durability of new materials that can withstand the operating conditions present in the modern aerojet or turbine engine. With these changes in engine materials, there has arisen a corresponding need to develop new repair methods appropriate for such materials.
The turbine blade is one engine component that directly experiences severe engine conditions. Turbine blades are thus designed and manufactured to perform under repeated cycles of high stress and high temperature. An economic consequence of such a design criteria is that turbine blades can be quite expensive. It is thus highly desirable to maintain turbine blades in service for as long as possible, and to return worn turbine blades to service, if possible, through acceptable repair procedures.
Turbine blades used in modern jet or gas turbine engines are frequently castings from a class of materials known as superalloys. The superalloys can include alloys with high levels of nickel and/or cobalt. In the cast form, turbine blades made from superalloys include many desirable physical properties such as high strength. Advantageously, the strength displayed by this material remains present even under stressful conditions, such as high temperature and high pressure, experienced during engine operation.
Inconel 713 is one such superalloy. It is a preferred material for the construction of turbine blades. Inconel 713 is a precipitation hardenable alloy. Nickel, alloyed with materials such as aluminum and titanium, develops high strength characteristics that are sustainable at high temperatures, the temperature range that engine designers now seek. The strength arises in part through the presence of a gamma prime (γ′) phase of material. One characteristic of Inconel 713 is the high degree of gamma prime in cast materials.
Disadvantageously, the superalloys generally, and Inconel 713 in particular, are very difficult to weld successfully with known welding techniques. Various methods have been developed and are described in the technical literature related to resurfacing, restoring, repairing, and reconditioning worn turbine blades and Z-notch faces. However each of these methods has shortcomings or limitations that significantly limits the usefulness of the method.
Turbine blades used in contemporary jet engines often include a shroud with Z-notches for low pressure application. The term Z-notch refers to a configuration of the turbine blade shroud in the shape of a “Z”. Neighboring blades interlock at the Z-notch areas. The Z-notch interlock provides turbine blades an additional degree of stiffness to offset the twisting forces that the blades experience. Z-notches also counterbalance harmful vibrational movements in the turbine blades. The Z-notches are points of contact between turbine blades, and the interlocking faces of Z-notches thus can experience wear and erosion. Consequently, over a period of time in operation the Z-notch wear surfaces of turbine blades may need to be repaired or resurfaced.
Traditional repair methods have proven unsatisfactory for the Inconel 713 material. For example, some known welding techniques heat the workpiece, the Z-notch area of a turbine blade, to high temperatures, temperatures sufficient to weld the alloy. However, at such a temperature, the turbine blade may experience heat cracking and fracturing, rendering the blade unusable for further engine service. Hence, it is desirable to find a repair method suitable for Inconel 713 that does not subject the workpiece matrix to heat-induced damage.
Other repair techniques include cladding of a matrix material with a hardsurfacing material or other materials with good weldability. Such a method is disadvantageous with respect to turbine blades manufactured of Inconel 713. In particular, a multi-material blade cannot be returned to service with other turbine blades that are single material blades because the mismatch in mechanical properties between adjacent blades may speed up material loss for blades with lesser properties. Further, the heating requirement to fuse a cladding material to Inconel 713 can subject the substrate material to excessive heating as in known welding techniques. Accordingly there is also a need for a repair method in which the repair material itself is the same as the substrate material, such as where the blade is made of Inconel 713 and the repair material is the same.
The option of throwing out worn turbine blades and replacing them with new ones is not an attractive alternative because the blades are expensive. A low pressure turbine blade made of Inconel 713 can be costly to replace, and a single stage in an engine may contain several dozen such blades. Moreover, a typical gas turbine engine can have multiple rows or stages of turbine blades. Consequently there is a strong financial benefit for acceptable repair methods for Inconel 713 turbine blades.
Hence, there is a need for a turbine repair method that addresses one or more of the above-noted drawbacks. Namely, a repair method is needed that does not result in significant component damage, and/or does not require the use of materials other than Inconel 713 or any other blade material, and/or requires minimal consumption of superalloy in the repair process, and by virtue of the foregoing is therefore less costly as compared to the alternative of replacing worn parts with new ones. The present invention addresses one or more of these needs.